Various processes have been provided for the removal of sulfur dioxide from gaseous streams, such as combustion gases from fossil fuel-fired power plants, so as to reduce acidic components discharged into the air and prevent pollution of the environment. Some such processes are preferable to others for various reasons, either economic reasons, ease of operation, dependability, efficiency, and other factors. All such processes, however, have one or more factors that could be improved upon.
An especially useful process for removing sulfur dioxide from flue gases is the wet process where the flue gas is contacted in an aqueous slurry of lime that contains an effective amount of magnesium ions to enhance the process. Such a process is commercially used and is generally described in U.S. Pat. Nos. 3,919,393, 3,919,394, 3,914,378, 4,976,937 and other publications. This process has been commercially proven and is economical, easy to operate, dependable and efficient. Such a process, however, does produce an aqueous sludge of primarily calcium sulfite which is difficult to dewater. In conventional operation of such a magnesium-enhanced lime scrubbing process, the moisture content of the sludge produced, after dewatering, is relatively high, with a solids content in the range of 25-48 percent by weight achieved, depending upon the design of the process and the conditions of the process operation.
Efforts have been made to provide for better dewatering of aqueous sludges produced by lime scrubbing processes that contain calcium sulfites. For example, U.S. Pat. No. 4,454,101 describes a method where a predetermined amount of thiosulfate ions are provided in a scrubbing loop that produces sulfite-containing liquor slurries where the presence of the thiosulfate ions is said to produce calcium sulfite hemihydrate in a tabular prismatic mode and substantially eliminates the occurrence of porous lamellar aggregates of randomly oriented micaceous crystallites of the calcium sulfite hemihydrate so as to increase the settling rate and enable the ultimate dewatering of the sludge to a high solids content by weight. Another process proposed to produce large, regularly shaped calcium sulfite solids in a calcium-based flue gas desulfurization system is described in U.S. Pat. No. 5,246,679, which provides for the addition of a chemical crystal modifier, preferably an organophosphonate, to the calcium sulfite-containing slurry produced by a flue gas desulfurization process under inhibited oxidation conditions. This method is said to produce large, thick, substantially uniformly sized calcium sulfite crystals that exceed significantly 50 microns. Other modifiers suggested include carboxylic acids, polyacrylates, sulfonates, organic phosphates and polyphosphates.